There is an LED illumination dimming circuit 100A that controls dimming of an LED element, such as those described in Japanese Patent Laid-Open Nos. 2009-123681 and 2012-59662 (FIG. 5).
The LED illumination dimming circuit 100A includes a resistor “R1”, a bipolar transistor “Q1” having a collector connected to a cathode of an LED element 4 having an anode connected to a power supply 2, a constant current circuit 6 connected between an emitter of a bipolar transistor “Q1” and a ground, and a microcomputer 5 that controls the bipolar transistor “Q1” and the constant current circuit 6 in response to a dimming signal output from a dimmer 1 (FIG. 5).
In a constant current control in a current critical mode, the LED illumination dimming circuit 100A controls the bipolar transistor “Q1” to lower a peak current to achieve dimming (linear dimming).
However, in the constant current control in the current critical mode, if the conventional LED illumination dimming circuit 100A described above lowers the peak current to achieve dimming (linear dimming), the switching frequency of the bipolar transistor “Q1” increases.
The increase of the switching frequency poses various problems, such as an increase of noise and a decrease of the efficiency.
The switching frequency may be lowered when dimming is not performed. However, the switching frequency cannot be lowered below an audio frequency, so that the actual dimming range is limited.
In addition, if the switching frequency is lowered when dimming is not performed, the size of components, such as a transformer, undesirably increases.
Furthermore, the conventional LED illumination dimming circuit 100A described above needs to have a signal terminal for receiving a signal that prescribes an on width in which the bipolar transistor “Q1” is turned on and a signal terminal for receiving a signal that prescribes an off width in which the bipolar transistor “Q1” is turned off.
That is, the conventional LED illumination dimming circuit 100A described above needs to have a plurality of terminals.